1. Field of the Invention
The present invention relates generally to a method and an apparatus for transmitting interference related control information in order to improve reception performance of a User Equipment (UE) which receives a downlink signal, in a cellular mobile communication system based on a Long Term Evolution-Advanced (LTE-A) system.
2. Description of the Related Art
From the early stage of providing voice-oriented services, a mobile communication system has evolved into a high-speed and high-quality wireless packet data communication system to provide data and multimedia services. Various mobile communication standards such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and Long Term Evolution-Advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) of the 3rd Generation Partnership Project-2 (3GPP2), and IEEE 802.16 have recently been developed to support high-speed and high-quality wireless packet data communication services. In particular, the LTE system, which is a system developed to efficiently support high speed wireless packet data transmission, maximizes wireless system capacity by using various wireless access technologies. The LTE-A system, which is a wireless system obtained by advancing the LTE system, has an improved data transmission capacity compared to the LTE system.
In general, the LTE refers to an evolved Node B (eNB) and a UE apparatus corresponding to Release 8 or 9 of the 3GPP standard organization and the LTE-A refers to an eNB and a UE apparatus corresponding to Release 10 of the 3GPP standard organization. The 3GPP standard organization has standardized the LTE-A system and is now discussing the standard for a subsequent Release with improved performance, based on the standardized LTE-A system.
The existing 3rd Generation (3G) and 4th Generation (4G) wireless packet data communication systems such as HSDPA, HSUPA, HRPD, and LTE/LTE-A employ an Adaptive Modulation and Coding (AMC) scheme, a channel-sensitive scheduling scheme, and the like to improve transmission efficiency.
When the AMC scheme is used, a transmitter can adjust the amount of transmission data depending on a channel state. That is, when a channel state is poor, a transmitter may adjust the error probability at the receiver to a desired level by increasing the data rate, and when a channel state is good, the transmitter may efficiently transmit at high data rates while adjusting the error probability at the receiver to a desired level. With the use of the channel-aware scheduling resource management method, the transmitter selectively provides a service to a user having a good channel state among a plurality of users, and thus the system capacity may increase as compared with the method of assigning a channel to one user and providing a service to the user with the assigned channel. Such a capacity increase as in the above description is referred to as “multi-user diversity gain”. In short, the AMC scheme and the channel-sensitive scheduling scheme are methods that allow a transmitter to apply an appropriate modulation and coding technique at a point of time that is determined to be most efficient based on partial channel state information fed back from a receiver.
When being used with the Multiple Input Multiple Output (MIMO) wireless system using the spatial transmission scheme (such as open loop, closed loop and the like), the AMC scheme, as described above, may include a function of determining the number of spatial layers or the rank of a transmitted signal. In this case, when determining an optimum data rate, the AMC scheme also determines how many layers are used for transmission using the MIMO, not simply considering only a coding rate and a modulating scheme.
The MIMO, which transmits a wireless signal using a plurality of transmission antennas, is classified into Single User MIMO (SU-MIMO) which performs transmission to one UE and a Multi User MIMO (MU-MIMO) which performs transmission to a plurality of UEs using the same time and frequency resource. This is also referred to as spatial division multiple access (SDMA). In the case of the SU-MIMO, a plurality of transmission antennas transmits a wireless signal to one receiver using a plurality of spatial layers. At this time, the receiver should include multiple reception antennas in order to support the multiple spatial layers. In contrast, in the case of the MU-MIMO, multiple transmission antennas transmit a wireless signal to multiple receivers using multiple spatial layers. The MU-MIMO is more advantageous than the SU-MIMO in that the MU-MIMO does not require a receiver equipped with a plurality of reception antennas. However, the MU-MIMO is disadvantageous in that, since wireless signals are transmitted to a plurality of receivers through the same frequency and time resource, interference (multi-user or inter-user interference) may occur between the wireless signals for different receivers.
Meanwhile, in recent years, researches have been actively conducted on switching the next generation system from the Code Division Multiple Access (CDMA), which is a multiple access scheme used in the 2nd generation and 3rd generation mobile communication system, to the Orthogonal Frequency Division Multiple Access (OFDMA). The 3GPP and 3GPP2 have started their standardizations on the evolved systems employing the OFDMA. It is generally known that the OFDMA scheme, as compared to the CDMA scheme, can expect a capacity increase. One of the several reasons causing the capacity increase in the OFDMA scheme is that the OFDMA scheme may perform scheduling in a frequency domain (Frequency Domain Scheduling). Although a capacity gain is acquired according to the time-varying channel characteristic using the channel-aware scheduling scheme, it is also possible to obtain a higher capacity gain with use of the frequency-varying channel characteristic.
FIG. 1 illustrates a time-frequency resource in an LTE/LTE-A system.
Referring to FIG. 1, a wireless resource, which an eNB transmits to a UE, is divided into a Resource Block (RB) unit on a frequency axis and is divided into a sub-frame unit on a time axis. In the LTE/LTE-A system, the RB generally includes 12 subcarriers and occupies a band of 180 kHz. In the LTE/LTE-A system, the sub-frame is generally configured by 14 OFDM symbol intervals and occupies a time interval of 1 msec. The LTE/LTE-A system may assign a resource in a subframe unit on the time axis and assign a resource in an RB unit on the frequency axis in performing scheduling.
FIG. 2 illustrates a wireless resource of one sub-frame and one RB, which is a a minimum unit schedulable in a downlink in the LTE/LTE-A system.
Referring to FIG. 2, a wireless resource includes one sub-frame on a time axis and one RB on a frequency axis. Such a wireless resource includes 12 subcarriers in a frequency region, includes 14 OFDM symbols in a time region, and thus includes 168 inherent frequencies and time positions. In the LTE/LTE-A, each inherent frequency and time position illustrated in FIG. 2 is referred to as a Resource Element (RE). Further, one sub-frame includes two slots, each of the two slots being configured by 7 OFDM symbols.
The following several different types of signals may be transmitted in the wireless resource illustrated in FIG. 2.
1. CRS (Cell Specific Reference Signal): Reference signal transmitted to all UEs belonging to one cell.
2. DeModulation Reference Signal (DMRS): reference signal transmitted to a specific UE.
3. Physical Downlink Shared CHannel (PDSCH): data channel transmitted via a downlink, which is used by an eNB to transmit traffic to a UE and is transmitted using an RE not used for reference signal transmission in the data region of FIG. 2.
4. Channel Status Information Reference Signal (CSI-RS): The CSI-RS is used in measuring a channel state of the reference signal transmitted to UEs belonging to one cell. A plurality of CSI-RSs may be transmitted to one cell.
5. Other control channels (Physical Hybrid ARQ Indicator Channel (PHICH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH)): control channels for providing control information required for a UE to receive a PDSCH or transmitting Acknowledgement (ACK)/Negative Acknowledgement (NACK) for operating Hybrid automatic repeat request (HARQ) for uplink data transmission.
In addition to the signals, the LTE-A system can configure muting such that the CSI-RS transmitted by another eNB can be received without interference by UEs of a corresponding cell. The muting can be applied to a position at which a CSI-RS can be transmitted, and a UE generally skips a corresponding wireless resource and receives a traffic signal. In the LTE-A system, the muting is also referred to as a zero-power CSI-RS as another term. This is because the muting is applied to a CSI-RS position and transmission power is not transmitted.
As illustrated in FIG. 2, the CSI-RS can be transmitted using a part of positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of antennas which transmit the CSI-RS. Further, the muting may be also applied to a part of the positions marked by A, B, C, D, E, F, G, H, I, and J. In particular, a CSI-RS may be transmitted to 2, 4, or 8 REs according to the number of transmission antenna ports. For example, in FIG. 2, the CSI-RS is transmitted to half of the specific pattern when the number of antenna ports is 2, the CSI-RS is transmitted to the entire specific pattern when the number of antenna ports is 4, and the CSI-RS is transmitted using two patterns when the number of antenna ports is 8. In contrast, in a case of the muting, the CSI-RS is always transmitted in one pattern unit. That is, the muting may be applied to a plurality of patterns, but cannot be applied to only a part of one pattern when a muting position does not overlap a CSI-RS position. However, when the CSI-RS positions overlap the muting positions, the muting can be applied to only a part of one pattern.
In a cellular system, a Reference Signal (RS) should be transmitted in order to measure a downlink channel state. In the case of the LTE-A system of the 3GPP, a UE measures a channel state between an eNB and the UE by using a CRS or a CSI-RS transmitted by the eNB. Several factors should be basically considered for the channel state, here, an amount of interference in the downlink is included. The amount of interference in the downlink includes interference signals, thermal noise, etc. generated by antennas belonging to a neighbor eNB, and is an important factor for a UE to determine a channel situation of the downlink. As an example, when an eNB having one transmission antenna transmits a signal to a UE having one reception antenna, the UE should determine an energy per one symbol which can be received via a downlink and an amount of interference to be simultaneously received from a section which receives the corresponding symbol on the basis of a reference signal which has been received from the eNB, so as to determine a Signal to Noise plus Interference Ratio (SNIR). The SNIR corresponds to a value obtained by dividing a power of a received signal by interference plus noise signal power. In general, a higher SNIR may result in better reception performance and a higher data rate (if single-user decoding is applied). The determined SNIR, a value corresponding thereto, or the maximum data rate supportable by the corresponding SNIR is reported to the eNB (also called channel quality indicator CQI), and thus the eNB can determine the data rate at which to transmit data to the UE via a downlink.
In a case of a general mobile communication system, an eNB equipment is disposed in a central point of each cell, and the corresponding eNB equipment communicates with a terminal (UE) using one or more antennas positioned in a limited place. A mobile communication system in which antennas belonging to one cell are arranged in the same location is referred to as a Centralized Antenna System (CAS). In contrast, a mobile communication system in which antennas (Remote Radio Heads; RRHs) belonging to one cell are located at distributed positions in the cell is called a Distributed Antenna System (DAS).
FIG. 3 illustrates an arrangement of antennas at distributed positions in a typical distributed antenna system.
Referring to FIG. 3, a DAS formed by two cells 300 and 310 is illustrated. The cell 300 is formed by one high-power antenna 320 and four low-power antennas 340. The high-power antenna 320 provides a minimum service to the entire area included in the cell area. In contrast, the low-power antennas 340 can provide a service based on a high data rate but only to UEs in a limited area within a cell. Further, the high-power antenna 320 and the low-power antennas 340 can operate according to scheduling and wireless resource allocation of a central controller while being connected to the central controller, as indicated by reference numeral 330. In the DAS, one or more antennas may be arranged at a location of one antenna which is geographically separated (one or more antennas may be co-located (antenna group) or distributed). In this way, in the present invention, in the DAS, an antenna or antennas arranged in the same location is called an antenna group (RRH group).
In the DAS as illustrated in FIG. 3, a UE receives a signal from one antenna group which is geographically separated, and a signal transmitted from other antenna groups acts as interference.
FIG. 4 illustrates an occurrence of interference in the case of transmission to different UEs according to each antenna group in a distributed antenna system.
Referring to FIG. 4, a first UE (UE1) 400 receives a traffic signal from an antenna group 410. In contrast, a second UE (UE2) 420 receives a traffic signal from an antenna group 430, a third UE (UE3) 440 receives a traffic signal from an antenna group 450, and a fourth UE (UE4) 460 receives a traffic signal from an antenna group 470. The UE1 400 receives a traffic signal from the antenna group 410 while receiving interference from the other antenna groups 430, 450, and 470 which transmit a traffic signal to the other UEs 420, 440, and 460, respectively. That is, a signal transmitted from the antenna groups 430, 450, and 470 may cause an interference effect to the UE1 400.
In general, interference generated by another antenna group in a distributed antenna system includes two types of interference as follows.                Inter-cell interference: Interference generated between antenna groups belonging to different cells.        Intra-cell interference: Interference generated between antenna groups belonging to the same cell.        
An example of intra-cell interference for the UE1 400 of FIG. 4 is interference generated in the antenna group 430 belonging to the same cell. Further, an example of inter-cell interference for the UE 400 is interference generated between the antenna groups 450 and 470 belonging to neighboring cells. The inter-cell interference and the intra-cell interference are received by a UE at the same time so as to disturb data channel reception of the UE and lowering the SNIR.
In general, when a UE receives a wireless signal, a desired signal is received together with noise and interference. That is, the reception signal may be expressed by Equation (1) as follows.r=s+noise+interference  (1)
In Equation (1), “r” denotes a reception signal, “s” denotes a transmission signal, “noise” denotes noise having the Gaussian distribution, and “interference” denotes an interference signal generated in a wireless communication system. The interference signal may be generated in the following situations.                Interference at neighboring transmission points: when a signal transmitted by a neighboring cell or a neighboring antenna in the DAS generates interference to a desired signal.        Interference at the same transmission point: when MU-MIMO transmission is performed at one transmission point using a plurality of antennas, when signals for different users generate interference therebetween.        
A value of the SNIR is changed according to the magnitude of the interference, thereby influencing reception performance. In general, the interference is a factor which causes most significantly system performance deterioration, and the system performance depends on how to appropriately control the interference. In order to control interference, various standard technologies for supporting coordinated multi-point (CoMP) transmission and reception, which is a type of collaborative communication, have been introduced in LTE and LTE-A. In the CoMP transmission, a network comprehensively and centrally controls transmission of a plurality of eNBs and transmission points so as to determine the magnitude of the interference and existence of the interference in a downlink and an uplink. As an example, when there are two eNBs, a central controller of the network can stop signal transmission from a second eNB (among the two eNBs) such that interference is not generated in a UE receiving a signal from a first eNB (among the two eNBs).
A wireless communication system performs forward error correction (FEC) coding in order to correct an error generated in a transmission/reception process. In the LTE/LTE-A system, a convolution code, a turbo code, etc. are used for the error correction coding. In order to improve decoding performance of the FEC coding, a receiver does not use a hard decision but uses a soft decision when decoding a modulated modulation symbol such as Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (QAM), and 64-QAM. All of the modulation mentioned schemes QPSK, 16 QAM and 64 QAM use complex symbols, e.g. two bits {(0,0), (0,1), (1,0), (1,1)}represent one QPSK symbol ((1,1,−1,−1)). When a transmission port transmits “+1” or “−1”, a receiver employing the hard decision selects and outputs either “+1” or “−1” for a received signal. In contrast, a receiver employing the soft decision outputs both information on which of “+1” and “−1” is received for a received signal and the reliability of the corresponding decision. Such reliability information may be used to improve decoding performance in the process of decoding.
A receiver employing soft decision generally uses a log likelihood ratio (LLR) to calculate a soft output value. When a Binary Phase Shift Keying (BPSK) modulation scheme is used in which the transmission signal is either “+1” or “−1”, the LLR is defined by Equation (2) as follows.
                              L          ⁢                                          ⁢          L          ⁢                                          ⁢          R                =                  log          ⁢                                    f              ⁡                              (                                                      r                    |                    s                                    =                                      +                    1                                                  )                                                    f              ⁡                              (                                                      r                    |                    s                                    =                                      -                    1                                                  )                                                                        (        2        )            
In Equation (2), “r” denotes a reception signal, and “s” denotes a transmission signal. This also applies for the higher order modulation schemes on a bit level, e.g., for both bits representing one QPSK symbol. Further, the conditional probability density function ƒ(r|s=+1) is a probability density function of the reception signal under a condition that “+1” is transmitted as the transmission signal. Likewise, the conditional probability density function ƒ(r|s=−1) is a probability density function of the reception signal under a condition that “−1” is transmitted as the transmission signal. For any other modulation such as QPSK, 16QAM, or 64QAM, an LLR may also be mathematically expressed in the same manner. The conditional probability density function has a Gaussian distribution when there is no interference.
FIG. 5 illustrates a conditional probability density function.
Referring to FIG. 5, a graph 500 corresponds to the conditional probability density function ƒ(r|s=−1), and a graph 510 corresponds to the conditional probability density function ƒ(r|s=+1). For example, when a value of a reception signal is identical to a value depicted by reference point 520, a receiver calculates an LLR as log(f2/f1) using such conditional probability density functions, wherein f1 is the function value of the probability density function 500 at the abscissa value 520 and wherein f2 is the function value of the probability density function 510 at the abscissa value 520. The conditional probability density functions as illustrated in FIG. 5 correspond also to cases where noise and interference are both modeled with the Gaussian distribution.
In a mobile communication system such as the LTE/LTE-A system, an eNB transfers several tens of bits or more of information to a UE in one PDSCH transmission. At this time, the eNB encodes information to be transmitted to the UE, modulates the encoded information in schemes such as QPSK, 16QAM, and 64AQM, and then transmits the modulated information. As a result, the UE, which has received the PDSCH, generates LLRs for several tens or more of encoded symbols (e.g. QPSK=2 bits, 16 QAM=4 bits, 64 QAM=6 bits) in the process of demodulating several tens or more of modulated symbols and transfers the generated LLRs to a decoder.
FIG. 6 illustrates a conditional probability density function when it is assumed that a reception signal is transmitted in the BPSK modulation scheme and an interference signal is also transmitted in the BPSK modulation scheme.
In general, noise samples are modeled with a Gaussian distribution, but interference may not Gaussian distributed depending on the situation. The representative reason that the interference is not Gaussian distributed is that the interference is a wireless signal for another receiver, which is unlike noise. That is, since “interference” in Equation (1) is a wireless signal for another receiver, the interference is transmitted in a state in which the modulation schemes such as the BPSK, the QPSK, the 16QAM, and the 64QAM are applied thereto. As an example, when an interference signal is modulated in the “BPSK”, the interference has a probability distribution having a value of one of “+k” and “−k” in the same probability. In the above, “k” is a value determined by a signal intensity attenuation effect of a wireless signal.
Meanwhile, in FIG. 6, it is assumed that noise accords with a Gaussian distribution.
The conditional probability density functions in FIG. 6 are different from the conditional probability density functions in FIG. 5. In FIG. 6, the curve 620 corresponds to a conditional probability density function ƒ(r|s=−1), and the curve 630 corresponds to a conditional probability density function ƒ(r|s=+1). Further, the amplitude of the shift 610 is determined according to the intensity of the interference signal and is determined according to the influence on a wireless signal. For example, when a value of a reception signal is identical to a value depicted by reference point 600, a receiver calculates an LLR as log(f4/f3) using such conditional probability density functions, wherein f3 is the function value of the probability density function 620 at the abscissa value 600 and wherein f4 is the function value of the probability density function 630 at the abscissa value 600. Since the conditional probability density function values are different from each other, the LLR has a value different from that of the LLR in FIG. 5. That is, the LLR obtained by considering a modulation scheme of an interference signal with a non-Gaussian distribution (as in FIG. 6) is different from the LLR calculated on the basis of an assumption that interference accords with the Gaussian distribution (as in FIG. 5).
FIG. 7 illustrates a conditional probability density function when it is assumed that a reception signal is transmitted in the BPSK modulation scheme and an interference signal is also transmitted in the 16QAM modulation scheme.
FIG. 7 illustrates that a conditional probability density function may change according to a difference in the modulation scheme of interference. In all examples illustrated in FIG. 6 and FIG. 7, a reception signal is transmitted in the BPSK modulation scheme. However, in FIG. 6, the interference corresponds to the BPSK, and in FIG. 7, the interference corresponds to the 16QAM. That is, even when the modulation schemes of a reception signal is identical to each other, the conditional probability density functions differ from each other according to the modulation schemes of an interference signal, and as a result, the calculated LLRs differ from each other.
As described in parts relating to FIGS. 5, 6, and 7, the LLR has different values according to how a receiver assumes and calculates the interference. In order to optimize reception performance, the LLR should be calculated using the conditional probability density function on which a statistical characteristic of an actual interference is reflected, i.e. the LLR calculation should depend on a statistical characteristic (e.g. Gaussian characteristic, BPSK modulated characteristic, 16QAM modulated characteristic or the like) of an actual interference. Further, the LLR should be calculated after an interference signal is cancelled from a reception signal in advance.
For example, when an interference signal is transmitted in the BPSK modulation scheme, the LLR should be calculated on the basis of the assumption that interference is transmitted from a receiver in the BPSK modulation scheme or the LLR should be calculated after interference modulated in the BPSK is cancelled. However, in a case where an interference is transmitted in the BPSK modulation scheme, when the LLR is calculated without an interference cancellation procedure on the basis of an assumption that the interference has the Gaussian distribution or is transmitted in a different modulation scheme such as the 16QAM modulation scheme from a receiver, a non-optimized LLR value is calculated, and thus, reception performance cannot be optimized.